High-strength cold rolled steel sheet having high hole expansion ratio, highstrength hot-dip galvanized steel sheet, and manufacturing methods therefor

ABSTRACT

Provided is a high-strength cold rolled steel sheet, a high-strength hot-dip galvanized steel sheet manufactured using the cold rolled steel sheet, and manufacturing methods therefor, the high-strength cold rolled steel sheet comprising, by wt %, 0.17-0.21% of carbon (C), 0.3-0.8% of silicon (Si), 2.7-3.3% of manganese (Mn), 0.3-0.7% of chromium (Cr), 0.01-0.3% of aluminum (Al), 0.01-0.03% of titanium (Ti), 0.001-0.003% of boron (B), 0.04% or less of phosphorus (P), 0.02% or less of sulfur (S), 0.01% or less of nitrogen (N) and the balance of iron (Fe) and other inevitable impurities, wherein the amounts of carbon (C), silicon (Si) and aluminum (Al) satisfy the following mathematical relation (1). [Mathematical relation (1)] [C]+([Si]+[Al])/5≤0.35% (wherein [C], [Si] and [Al] respectively mean the wt % of C, Si and Al.)

TECHNICAL FIELD

The present disclosure relates to a high-strength cold rolled steelsheet and high-strength hot-dip galvanized steel sheet having a highhole expansion ratio, and a manufacturing method thereof.

BACKGROUND ART

In recent years, development of a technology of manufacturing a steelsheet having high strength has been promoted to reduce the weight ofautomobiles. A steel sheet having both high strength and formability mayincrease productivity, so it is excellent in terms of economy and ismore advantageous in terms of safety of final parts. In particular,demand for steel having high tensile strength (TS) of 1180 MPa or higherhas increased because a steel sheet having high tensile strength (TS)has a high bearing load until fracture occurs. In the related art, manyattempts have been made to improve strength of the existing steel, butit was found that simple improvement of the strength degrades ductilityand hole expansion ratio (HER). Meanwhile, transformation inducedplasticity (TRIP) steel sheet in which a large amount of Si or Al isadded may be a related art which overcomes the aforementionedshortcomings. However, in the case of TRIP steel sheet, it is possibleto obtain an elongation of 14% or more at TS 1180 MPa class but liquidmetal embrittlement (LME) resistance is deteriorated due to the additionof a large amount of Si and Al, which leads to poor weldability, andthus, commercialization of TRIP steel sheet as a material for automobilestructures is limited.

In addition, various yield ratios are pursued in the same tensilestrength class according to usages and purposes, and it is not easy toproduce a steel having high hole expansion ratio with a steel sheethaving a low yield ratio. The reason is because it is usually necessaryto introduce a martensite or ferrite phase as a second phase to lower ayield ratio but such a structural characteristics is a factor thatimpairs the hole expansion ratio.

Patent document 1 discloses a high-strength cold rolled steel sheethaving yield ratio, strength, hole expansion ratio, delayed fractureresistance characteristics and having a high elongation of 17.5% ormore. However, Patent document 1 has a disadvantage in that weldabilityis poor due to an occurrence of LME due to a high Si addition.

Therefore, the present disclosure proposes a 1180 MPa-class steelmaterial exhibiting high strength and excellent hole expansion ratio of25% or more, elongation of 5% to 13%, and excellent weldability even ata low yield ratio, and a manufacturing method thereof.

RELATED ART DOCUMENT

-   (Patent document 1) Korean Patent Laid-open Publication No.    2017-7015003

DISCLOSURE Technical Problem

An aspect of the present disclosure may provide a high-strength coldrolled steel sheet having elongation suitable for machining, a high holeexpansion ratio, and good weldability, while having a high strength anda low yield ratio, a high-strength hot-dip galvanized steel sheetmanufactured using the same, and a manufacturing method thereof.

Technical objects to be achieved by the present invention are notlimited to the aforementioned technical objects, and other technicalobjects not described above may be evidently understood by a personhaving ordinary skill in the art to which the present invention pertainsfrom the following description.

Technical Solution

According to an aspect of the present disclosure, a high-strength coldrolled steel sheet may include, by weight percent (wt %), 0.17 to 0.21%of carbon (C), 0.3 to 0.8% of silicon (Si), 2.7 to 3.3% of manganese(Mn), 0.3 to 0.7% of chromium (Cr, 0.01 to 0.3% of aluminum (Al), 0.01to 0.03% of titanium (Ti), 0.001 to 0.003% of boron (B), 0.04% or lessof phosphorus (P), 0.02% or less of sulfur (S), 0.01% or less ofnitrogen (N), the balance of iron. (Fe), and other inevitableimpurities, wherein the contents of carbon (C), silicon (Si), andaluminum (Al) satisfy Equation 1 below, a microstructure thereofincludes (by area fraction, 3 to 7 of retained to 15% of freshmartensite, 5 or less (including 0%) of ferrite, and the balance ofbainite or tempered martensite, and, by volume fraction, 1 to 3% of acementite phase, as a second phase, is precipitated and distributedbetween bainite laths or in the laths or grain boundary of a temperedmartensite phase,

[C]+([Si]+[Al])/5≤0.35%  [Equation (1)]

wherein [C], [Si], [Al] refer to weight percents of C, Si, and Al,respectively.

The cold rolled steel sheet may further include 0.1% or less of copper(Cu), 0.1% or less of nickel (Ni), and 0.1% or less of molybdenum (Mo).

The cold rolled steel sheet may further include 0.03% or less of niobium(Nb) and 0.01% of less of vanadium (V).

The cold rolled steel sheet may have a tensile strength of 1180 MPa ormore, a yield ratio of 0.65 to 0.85, a hole expansion ratio of 25% ormore (HER), and an elongation of 5 to 13%.

According to another aspect of the present disclosure, a high strengthhot-dip galvanized steel sheet may further include a hot-dip zincplating layer on a surface of the high-strength cold rolled steel sheetdescribed above.

The high strength hot-dip galvanized steel sheet may further include analloyed hot-dip zinc plating layer on a surface of the high-strengthcold rolled steel sheet described above.

According to another aspect of the present disclosure, a method ofmanufacturing a high-strength cold rolled steel sheet may include:preparing a slab including, by weight percent (wt %), 0.17 to 0.21% ofcarbon (C), 0.3 to 0.8% of silicon (Si), 2.7 to 3.3% of manganese (Mn),0.3 to 0.7% of chromium (Cr, 0.01 to 0.3% of aluminum (Al), 0.01 to0.03% of titanium (Ti), 0.001 to 0.003% of boron (B), 0.04% or less ofphosphorus (P), 0.02% or less of sulfur (S), 0.01% or less of nitrogen(N), the balance of Iron (Fe), and other inevitable impurities, whereinthe contents of carbon (C), silicon (Si), and aluminum (Al) satisfyEquation A below; heating the slab to a temperature in a range of 1,150°C. to 1,250° C.; finish hot rolling the heated slab within finishdelivery temperature (FDT) range of 900° C. to 980° C.; cooling the slabat an average cooling rate of 10° C./sec to 100° C./sec after the finishhot rolling; coiling the slab in a temperature in a Lange of 500° C. to700° C.; rolling the slab at a cold-rolling reduction ratio of 30% to60% to obtain a cold rolled steel sheet; continuously annealing the coldrolled steel sheet at a temperature in a range of (Ae3+30° C. to Ae3+80°C.); primarily cooling the continuously annealed steel sheet at anaverage cooling rate of 10° C./s or less to a temperature in a range of560° C. to 700° C. and secondarily cooling the steel sheet at an averagecooling rate of 10° C./s or more to a temperature in a range of 270° C.to 330° C.; and reheating the cooled steel sheet at a temperatureincrease rate of 5° C./s or lower to a temperature in a range of 380° C.to 460° C.

[C]+([Si]+[Al])/5≤0.35%  [Equation (1)]

wherein [C], [Si], and [Al] refer to weight percent of C, Si, and Al,respectively.

The slab may further include 0.1% or less of copper (Cu), 0.1% or lessof nickel (Ni), and 0.1% or less of molybdenum (Mo).

The slab may further include 0.03% or less of niobium (Nb) and 0.01% orless of vanadium (V).

The continuous annealing may be performed at a temperature in a range of830° C. to 880° C.

According to another aspect of the present disclosure, a method ofmanufacturing a high strength hot-dip galvanized steel sheet may furtherinclude: performing hot-dip zinc plating on the reheated cold rolledsteel sheet at a temperature in a range of 430° C. to 490° C.

After the hot-dip zinc plating, annealing for alloying may be performed,and cooling may then be performed to room temperature.

After cooling to the room temperature, temper rolling less than 1% maybe performed.

Advantageous Effects

According to exemplary embodiments in the present disclosure, ahigh-strength cold rolled steel sheet and hot-dip galvanized steel sheethaving a high hole expansion ratio of 25% or more and an elongation of5% to 13%, while having high tensile strength of 1180 MPa or more and alow yield ratio of 0.65 to 0.85, may be provided.

In addition, the high-strength hot-dip galvanized steel sheet of thepresent disclosure has characteristics that exhibit excellentweldability due to excellent LME resistance after galvanizing.

BEST MODE

The terminology used herein is for reference only to specificembodiments and is not intended to limit the present disclosure.Singular forms as used herein also include plural forms unless obviouslyindicate otherwise.

As used in the disclosure, the meaning of “including” specifies aspecific characteristics, regions, integers, steps, operations, elementsand/or components, and do not exclude presence or addition of otherspecific characteristics, regions, integers, steps, operations,elements, components and/or groups.

Unless indicated otherwise, it is to be understood that all the termsused in the specification, including technical and scientific terms havethe same meaning as those that are understood by those skilled in theart to which the present invention pertains. It must be understood thatthe terms defined by the dictionary are identical with the meaningswithin the context of the related art, and they should not be ideally orexcessively formally defined unless the context clearly dictatesotherwise.

Hereinafter, a high-strength cold rolled steel sheet and a high strengthhot-dip galvanized steel sheet according to an aspect of the presentdisclosure will be described in detail.

First, an alloy composition of a high-strength cold rolled steel sheetprovided in the present disclosure will be described in detail. In thiscase, the content of each component refers to weight % unless otherwisespecified.

Carbon (C): 0.17 to 0.21%

Carbon is a basic element that supports strength of steel through solidsolution strengthening and precipitation strengthening. If the amount ofcarbon is less than 0.17%, it is difficult to obtain strength equivalentto tensile strength (TS) of 1180 MPa, while satisfying other materials.Meanwhile, if the amount of carbon exceeds 0.21%, weldabilitydeteriorates and a target hole expansion ratio value cannot be obtained.Therefore, in the present disclosure, the content of carbon ispreferably limited to 0.17 to 0.21%. A lower limit of C is morepreferably 0.18%, and an upper limit of C is more preferably 0.20%.

Silicon (Si): 0.3 to 0.8%

Silicon is a key element of transformation induced plasticity (TRIP)steel that acts to increase a retained austenite fraction and elongationby inhibiting precipitation of cementite in a bainite region. If siliconis less than 0.3%, the elongation is too low as retained austeniterarely remains. Meanwhile, if silicon exceeds 0.8%, deterioration ofphysical properties of a weld portion due to formation of LME crackscannot be prevented and surface characteristics and plating propertiesof the steel deteriorate. Therefore, in the present disclosure, thecontent of silicon is preferably limited to 0.3 to 0.8%. A lower limitof Si is more preferably 0.4% and an upper limit of Si is morepreferably 0.6%.

Manganese (Mn): 2.7 to 3.3%

In the present disclosure, the amount of manganese may be 2.7 to 3.3%.If the manganese content is less than 2.7%, it is difficult to securestrength, and if the manganese content exceeds 3.3%, a bainitetransformation rate is slowed to form too much fresh martensite, makingit difficult to obtain high hole expansion ratio. In addition, if thecontent of manganese is high, a start temperature of martensiteformation is lowered and a cooling end temperature required to obtain aninitial martensite phase in an annealing water cooling step is too low.Therefore, in the present disclosure, the content of manganese ispreferably limited to 2.7 to 3.3%. A lower limit of Mn is morepreferably 2.8% and an upper limit of Mn is more preferably 3.1%.

Chromium (Cr): 0.3 to 0.7%

In the present disclosure, the amount of chromium may be 0.3 to 0.7%. Ifthe amount of chromium is less than 0.3%, it is difficult to obtaintarget tensile strength, and the amount of chromium exceeds an upperlimit of 0.7%, a transformation speed of bainite is slow, making itdifficult to obtain a high hole expansion ratio. Therefore, in thepresent disclosure, the content of chromium is preferably limited to 0.3to 0.7%. A lower limit of Cr is more preferably 0.4% and an upper limitof Cr is more preferably 0.6%.

Aluminum (Al): 0.01 to 0.3%

In the present disclosure, the amount of aluminum may be 0.01 to 0.3%.If the amount of aluminum is less than 0.01%, the steel may not besufficiently deoxidized and cleanliness is impaired. Meanwhile, theamount of aluminum exceeds 0.3%, castability of the steel isdeteriorated. Therefore, in the present disclosure, the content ofaluminum is preferably limited to 0.01 to 0.3%. A lower limit of Al ismore preferably 0.03% and an upper limit of Al is more preferably 0.2%.

Titanium (Ti): 0.01 to 0.03%, Boron (B): 0.001 to 0.003%

In the present disclosure, 0.01 to 0.03% of titanium and 0.001 to 0.003%of boron may be added to increase hardenability of the steel. If thecontent of titanium is less than 0.01%, boron may be bonded to nitrogen,so that the effect of strengthening hardenability of boron is lost, andthe content of titanium exceeds 0.03%, castability of the steel isdeteriorated. Meanwhile, if the content of boron is less than 0.001%, aneffective hardenability strengthening effect cannot be obtained, and ifboron is contained in excess of 0.003%, a boron carbide may be formed,significantly impairing hardenability. Therefore, in the presentdisclosure, the content of titanium is preferably limited to 0.01 to0.03%, and the content of boron is preferably limited to 0.001 to0.003%. A lower limit of Ti is more preferably 0.015% and an upper limitof Ti is more preferably 0.025%. A lower limit of B is more preferably0.015% and an upper limit of B is more preferably 0.0025%.

Phosphorus (P): 0.04% or Less

Phosphorus exists as an impurity in the steel and it is advantageous tocontrol its content as low as possible, but phosphorus is alsointentionally added to increase strength of the steel. However, if thephosphorus is excessively added, toughness of the steel may bedeteriorated. Therefore, in order to prevent this, in the presentdisclosure, an upper limit may be preferably limited to 0.04%. Morepreferably, the content of P is 0.01% or less.

Sulfur (S): 0.02% or Less

Like phosphorus, sulfur exists as an impurity in the steel, and it isadvantageous to control its content as low as possible. In addition,since sulfur deteriorates ductility and impact properties of the steel,an upper limit is preferably limited to 0.02% or less. The content of Sis more preferably 0.003% or less.

Nitrogen (N): 0.01% or Less

In the present disclosure, nitrogen is included in the steel as animpurity, and it is advantageous to control the content of nitrogen aslow as possible. If a large amount of nitrogen is added, an excessiveamount of nitride may be formed to degrade rollability due to excessivestructure refinement, to make it impossible to control a targetstructure, and to impair final quality such as impact characteristics,etc. Therefore, an upper limit thereof is preferably limited to 0.01% orless. The content of N is more preferably 0.0060% or less.

In addition to the aforementioned alloy composition, the steel sheet ofthe present disclosure may additionally include 0.1% or less of copper(Cu), 0.1% or less of nickel (Ni), and 0.1% or less of molybdenum (Mo).

Copper (Cu): 0.1% or Less, Nickel (Ni): 0.1% or Less, Molybdenum (Mo):0.1% or Less

Copper, nickel, and molybdenum are elements that increase strength ofsteel and are included as optional components in the present disclosure,and an upper limit of addition of each element is limited to 0.1%. Theseelements increase strength and hardenability of steel, but addition ofan excessive amount thereof may exceed a target strength class, andsince they are expensive elements, an upper limit of their addition ispreferably limited to 0.1% in terms of economical efficiency. Meanwhile,since copper, nickel and molybdenum act as solid solution strengthening,an addition thereof less than 0.03% may be too insignificant to achievesolid solution strengthening effect, and therefore, when copper, nickeland molybdenum are added, a lower limit thereof may be limited to 0.03%or more. An upper limit of each of Cu, Ni, and Mo is preferably 0.06%.

In addition to the alloy composition described above, the steel sheet ofthe present disclosure may additionally include 0.03% or less of niobium(Nb) and 0.01% or less of vanadium (V).

Niobium (Nb): 0.03% or Less, Vanadium (V): 0.01% or Less

Niobium and vanadium are elements that increase yield strength of steelthrough precipitation hardening and may be optionally added to increaseyield strength in the present disclosure. However, excessive contentthereof may significantly lower elongation and cause brittleness of thesteel, and thus, in the present disclosure, upper limits of niobium andvanadium are limited to 0.03% and 0.01% or less, respectively.Meanwhile, since niobium and vanadium cause precipitation hardening,even a small amount of addition thereof may be effective, but if niobiumand vanadium is added less than 0.005%, the effect may be insignificant.Therefore, when niobium and vanadium is added, a lower limit thereof maybe limited to 0.005% or more. Upper limits of Nb and V are preferably0.02% and 0.008%, respectively.

[C]+([Si]+[Al])/5≤0.35%  [Equation (1)]

wherein [C], [Si], [Al] refer to weight percents of C, Si, and Al,respectively.

In addition to the aforementioned contents of C, Si and Al, C, Si, andAl satisfy Equation (1) above. Liquid metal embrittlement (LME) ofplated steel occurs as liquid zinc penetrates an austenite grainboundary when tensile stress is formed at an austenite grain interfaceof the steel sheet in a state in which plated zinc becomes liquid duringspot welding. Since the LME phenomenon is particularly severe in thesteel sheet to which Si and Al are added, an addition amount of Si andAl is limited through Equation (1) above in the present disclosure. Inaddition, if the C content is high, an A3 temperature of the steel islowered to cause an austenite region vulnerable to LME to be expandedand weaken toughness of the material, and thus, the addition amount of Cis limited through Equation (1) above.

If the value of Equation (1) exceeds 0.35%, LME resistance isdeteriorated during spot welding as described above, and thus, there areLME cracks after the spot welding, which impairs fatigue characteristicsand structural safety. Meanwhile, as the value of Equation (1) issmaller, spot weldability and LME resistance are improved, so a lowerlimit thereof may not be separately set. However, if the value is lessthan 0.20%, it may be difficult to obtain high tensile strength of 1180MPa class together with an excellent hole expansion ratio although spotweldability and LME resistance are improved. Therefore, the lower limitmay be set to 0.25%.

The remaining component of the present disclosure is iron (Fe). In otherordinary steel manufacturing processes, unintended impurities mayinevitably be mixed from raw materials or a surrounding environment.Since these impurities are known to anyone of ordinary skill in thesteel manufacturing process, all the contents thereof are notspecifically mentioned in the present disclosure.

Meanwhile, the high-strength cold rolled steel sheet of the presentdisclosure that satisfies the aforementioned steel composition has amicrostructure including, by area fraction, 3 to 7% of retainedaustenite, 5 to 15% of fresh martensite, 5% or less (including 0%) offerrite, and the balance of bainite or tempered martensite, and acementite phase, as a second phase, is precipitated and distributed in abainite lath boundary or in a lath or grain boundary of the temperedmartensite, and a volume fraction thereof may be 1 to 3%.

In the high-strength cold rolled steel sheet according to the presentdisclosure, part of cementite precipitates and grows in themicrostructure by limiting the content of Si and Al that stabilizesaustenite by inhibiting the growth of cementite, by the condition ofEquation (1). This cementite is precipitated in a martensite lath orgrain boundary when martensite formed by secondary cooling is reheated,or is formed in a portion in which carbon is concentrated betweenbainite ferrite laths when bainite transformation occurs duringreheating after secondary cooling. In the cold rolled steel sheetaccording to the present disclosure, cementite having a volume fractionof 1% or more is precipitated by limiting the upper limits of Si and Alby Equation (1), but, nevertheless, austenite remains due to thepresence of partial Si and Al and carbon is distributed inside theretained austenite, and thus, the amount of precipitated cementite isless than 3%. In addition, since Si and Al are added to some extent,austenite remains present in the steel of the present disclosure at alevel of 3 to 7%, but a high fraction of retained austenite as intypical TRIP steels with very high Si and Al content is not distributedin the steel of the present disclosure.

In addition, in the present disclosure, fresh martensite structure isintroduced at a level of 5 to 15% to obtain a low yield ratio. If anaustenite phase fraction is high after the secondary cooling andreheating, the carbon content in the austenite is low and stability isinsufficient, and part of the austenite is transformed into freshmartensite in a subsequent cooling process, resulting in a lower yieldratio.

In addition, in the present disclosure, the ferrite structure is notdesirable for the hole expansion ratio, but may exist at a level of 5%or less (including 0%) during the manufacturing process. In addition,the balance in the microstructure of the present disclosure includesbainite or tempered martensite structure.

By having the alloy components and microstructure as described above,the high-strength cold rolled steel sheet of the present disclosureexhibits a high hole expansion ratio of 25% or more even at a tensilestrength of 1180 MPa or more and a low yield ratio of 0.65 to 0.85. Asdescribed above, the low yield ratio of the high-strength cold rolledsteel sheet according to the present disclosure is due to theintroduction of fresh martensite. The inventors of the presentapplication found that more than 25% or more hole expansion ratio evenwith the fresh martensite under the alloy component and the structurecontrol condition according to the present disclosure. In addition,since the high-strength cold rolled steel sheet according to the presentdisclosure limits the content of Si and Al, the TRIP effect is weak andan elongation of 5 to 13% is shown.

The present disclosure may also provide a hot-dip galvanized steel sheetobtained by performing a hot-dip galvanizing on a surface of thehigh-strength cold rolled steel sheet and an alloyed hot-dip galvanizedsteel sheet obtained by performing annealing for alloying on the hot-dipgalvanized steel sheet.

Next, a method of manufacturing a high-strength cold rolled steel sheetand a high strength hot-dip galvanized steel sheet according to anotheraspect of the present disclosure will be described in detail.

The high-strength cold rolled steel sheet according to the presentdisclosure may be manufactured by undergoing heating a steel slabsatisfying the steel component composition described above—hotrolling—cooling—coiling—cold rolling—continuous annealing—primary andsecondary cooling—reheating, and details thereof are as follows.

Steel Slab Preparation and Heating Process

First, a slab having the aforementioned alloy composition and satisfyingEquation (1) is prepared and heated to a temperature of 1150° C. to1250° C. Here, if a slab temperature is less than 1150° C., it may beimpossible to perform a next step, hot rolling. Meanwhile, if the slabtemperature exceeds 1250° C., a lot of energy is unnecessarily requiredto increase the slab temperature. Therefore, a heating temperature ispreferably limited to a temperature of 1150° C. to 1250° C. A lowerlimit of the heating temperature is more preferably 1190° C. and anupper limit of the heating temperature is more preferably 1230° C.

Hot Rolling Process

The heated slab is hot-rolled to a thickness suitable for an intendedpurpose under the condition that a finish delivery temperature (FDT) is900° C. to 980° C. If the FDT is lower than 900° C., a rolling load islarge and shape defects increase, resulting in poor productivity.Meanwhile, if the FDT exceeds 980° C., surface quality deteriorates dueto an increase in oxides due to an excessive high-temperature operation.Therefore, hot rolling is preferably performed under the condition thatthe FDT is 900° C. to 980° C. A lower limit of the FDT is morepreferably 910° C. and an upper limit of the FDT is more preferably 950°C.

Coiling Process and Cold Rolling Process

The hot-rolled steel sheet is cooled to a coiling temperature at anaverage cooling rate of 10° C./s to 100° C./s, and coiling is performedin a typical temperature in a range of 500° C. to 700° C. After coiling,the hot-rolled steel sheet is rolled at a cold-rolling reduction ratioof 30% to 60% to obtain a cold rolled steel sheet. If the averagecooling rate is less than 10° C./s, hot rolling productivity may beexcessively deteriorated, and if it exceeds 100° C./s, strength of anedge portion increases, thereby increasing a material deviation in awidth direction. A lower limit of the average cooling rate is preferably20° C./s and an upper limit of the average cooling rate is preferably80° C./s. A lower limit of a temperature for coiling is preferably 550°C. and an upper limit of the temperature for coiling is preferably 650°C. If the cold rolling reduction ratio is less than 30%, it may bedifficult to secure target thickness accuracy and it may be difficult tocorrect a shape of the steel sheet. Meanwhile, if the cold-rollingreduction rate exceeds 60%, a possibility of cracking at the edge of thesteel sheet may increase and the cold-rolling load is excessively large.Therefore, in the present disclosure, the cold rolling reduction rate atthe cold rolling step is preferably limited to 30 to 60%. A lower limitof the cold-rolling reduction ratio is more preferably 35% and an upperlimit of the cold-rolling reduction ratio is more preferably 50%.

Continuous Annealing Process

In the present disclosure, the cold rolled steel sheet is subjected tocontinuous annealing in a temperature in a range of (Ae3+30° C. toAe3+80° C.). More preferably, continuous annealing may be performed in atemperature in a range of 830° C. to 880° C. In addition, the continuousannealing may be carried out in a continuous alloying hot-dip platingfurnace. The continuous annealing step is to form austenite close to100% by heating up to a single phase of austenite and use the austenitefor subsequent phase transformation. If the continuous annealingtemperature is lower than Ae3+30° C. or less than 830° C., sufficientaustenite transformation is not performed, so that desired martensiteand bainite fractions cannot be secured after annealing. Meanwhile, ifthe continuous annealing temperature exceeds Ae3+80° C. or 880° C.,productivity may decrease and coarse austenite may be formed, resultingin material deterioration. In addition, oxides may grow duringannealing, making it difficult to secure surface quality of a platedmaterial. Ae3 may be calculated using thermodynamic software utilizing acomputer coupling of phase diagrams and thermochemistry (CALPHAD) methodcommonly used in the art.

Primary and Secondary Cooling Process

The continuously annealed steel sheet is primarily cooled at an averagecooling rate of 10° C./s or less up to a temperature in a range of 560to 700° C., and secondarily cooled at an average cooling rate of 10°C./s or more up to a temperature in a range of 270 to 330° C. tointroduce martensite. Here, a temperature for terminating the primarycooling may be defined as a time point at which rapid cooling is startedby additionally applying a quenching facility that has not been appliedin the primary cooling. When a cooling process is divided into primaryand secondary cooling and is carried out by stages, a temperaturedistribution of the steel sheet may become uniform in a slow coolingstep to reduce a final temperature and material deviation and it is alsoadvantageous to obtain a required phase composition.

The primary cooling is slow cooling at an average cooling rate of 10°C./s or less, and a cooling end temperature thereof may be in atemperature in a range of 560 to 700° C. If the primary cooling endtemperature is lower than 560° C., a ferrite phase is excessivelyprecipitated to deteriorate a final hole expansion ratio. Meanwhile, ifit exceeds 700° C., the secondary cooling is excessively loaded and aplate speed of the continuous annealing line has to be slowed, resultingin lower productivity. A lower limit of the primary cooling endtemperature is more preferably 580° C. and an upper limit of the primarycooling end temperature is more preferably 670° C.

For the secondary cooling, a quenching facility not applied in theprimary cooling may be additionally applied, and a hydrogen quenchingfacility using H₂ gas may be used. Here, it is important to control asecondary cooling end temperature to 270 to 330° C. at which anappropriate initial martensite fraction may be obtained. If thesecondary cooling end temperature is lower than 270° C., the initialmartensite fraction transformed during the secondary cooling is toohigh, so that there is no space for obtaining various phasetransformations required in a subsequent process and a shape andworkability of the steel sheet deteriorate. Meanwhile, if the secondarycooling end temperature exceeds 330° C., the initial martensite fractionis low and high hole expansion ratio cannot be obtained. A lower limitof the secondary cooling end temperature is more preferably 290° C. andan upper limit of the secondary cooling end temperature is morepreferably 320° C. If the average cooling rate during the secondarycooling is less than 10° C./s, a ferrite/bainite phase or the like maybe formed during cooling, resulting in a decrease in strength and makingit difficult to finally secure a desired microstructure.

Reheating Process and Hot-Dip Galvanizing Process

The cooled steel sheet is reheated at a temperature in a range of 380°C. to 460° C. at a temperature increase rate of 5° C./s or less totemper the martensite obtained in the previous step, induce bainitetransformation, and concentrate carbon in untransformed austeniteadjacent to bainite. Here, it is important to control a reheatingtemperature to 380 to 460° C., and if reheating temperature is lowerthan 380° C. or exceeds 460° C., the amount of phase transformation ofbainite is small, so too much fresh martensite is formed in a finalcooling process, significantly hurting elongation and hole expansionratio. A lower limit of the reheating temperature is more preferably440° C. and an upper limit of the reheating temperature is morepreferably 440° C. When the temperature increase rate during reheatingexceeds 5° C./s, tempering of the martensite phase formed during thesecondary cooling may be insufficient and there may be a possibility ofnot sufficiently obtaining bainite phase transformation during thetemperature increase.

After reheating, hot-dip galvanizing may be performed at a temperaturein a range of 430° C. to 490° C., annealing for alloying may beperformed as necessary, and then cooling may be performed to roomtemperature. Thereafter, a process of performing temper rolling lessthan 1% may be performed to correct the shape of the steel sheet andadjust yield strength.

MODE FOR INVENTION Example

Hereinafter, the present disclosure will be described in more detailthrough examples. However, it should be noted that the followingexamples are for illustrative purposes only and are not intended tolimit the scope of the present disclosure. This is because the scope ofthe present disclosure is determined by matters described in the claimsand matters reasonably inferred therefrom.

After preparing the slab having the alloy composition of Table 1, a coldrolled steel sheet was manufactured through heating the steel slab underthe conditions described in Tables 2 and 3—hotrolling—cooling—coiling—cold rolling—continuous annealing—primary andsecondary cooling—reheating. Meanwhile, the FDT shown in Tables 2 and 3below refer to a finish delivery temperature, CT refers to a hot-rolledcoiling temperature, SS refers to a continuous annealing temperature,SCS refers to a primary cooling end temperature, RCS refers to secondarycooling end temperature, and RHS refers to a reheating temperature.

After measuring a microstructure, mechanical properties, and maximum LMEcrack size for the prepared cold rolled steel sheet, the results areshown in Table 3 below.

As for the maximum LME crack size is, a sample was spot-welded undersevere conditions of dome radius 6 mm, pressing force 3.54 kN, weldingtime 234 ms, H/T 100 ms, tilting 5 degrees, and gap 1.0 mm, a certaincross section across a nugget was taken, and a maximum length of anexisting LME crack was then measured.

A type and fraction of the microstructure were measured through XRD peakanalysis in the case of retained austenite, and the fractions of theremaining fresh martensite, ferrite, cementite, bainite and temperedmartensite phase were measured through a scanning electron microscopeEBSD analysis.

TABLE 1 Alloy composition (wt %) C + Steel (Si + type C Si Mn Cr Al Ti BP S Cu Ni Mo Nb V N Al)/5 A  0.17  0.726  2.58  0.499 0.053  0.0190.0018 0.009 0.006 0.02 0.00 0.058 0.003 0.003 0.0045 0.33 B  0.173 0.544  2.76  0.018 0.052  0.019 0.0019 0.010 0.003 0.01 0.02 0.0620.004 0.002 0.0065 0.29 C  0.17  0.525 2.6 0.5 0.205  0.019 0.0019 0.0060.002 0.03 0.01 0.063 0.001 0.005 0.0034 0.32 D  0.162  0.501 2.5  0.470.450  0.018 0.0018 0.007 0.004 0.02 0.01 0.062 0.001 0.003 0.0077 0.35E  0.155  0.74  2.66  0.52 0.045  0.02 0.0019 0.007 0.004 0.01 0.020.061 0.004 0.005 0.0090 0.31 F  0.237  0.696 2.4  0.48 0.043  0.0180.0018 0.009 0.003 0.01 0.01 0.057 0.001 0.004 0.0088 0.38 G  0.182 0.72  3.58  0.514 0.048  0.022 0.0021 0.012 0.004 0.05 0.01 0.008 0.0030.003 0.0072 0.34 H  0.18  0.73  1.67  2.560 0.053  0.02 0.002  0.0060.003 0.02 0.01 0.015 0.001 0.001 0.0047 0.34 I  0.184  0.74  2.87 0.502 0.053  0.02 0.0019 0.006 0.006 0.03 0.01 0.005 0.004 0.004 0.00420.34 J  0.181  0.72  3.17  0.492 0.050  0.02 0.0021 0.004 0.006 0.030.01 0.006 0.002 0.002 0.0067 0.34 K 0.2  0.512  2.95  0.506 0.200 0.023 0.0022 0.008 0.004 0.04 0.01 0.008 0.002 0.004 0.0064 0.34 L 0.184  0.52  3.14  0.494 0.202  0.019 0.0020 0.007 0.003 0.01 0.000.009 0.002 0.004 0.0054 0.33 M  0.177  1.54  2.63  0.51 0.055  0.0220.0022 0.009 0.003 0.01 0.00 0.057 0.003 0.002 0.0046 0.50

TABLE 2 Average cooling Cold Slab Hot rate after Cold rolling heatingrolled hot rolled reduction Steel temperature thickness FDT rolling CTthickness ratio Classification type (° C.) (mm) (° C.) (° C./s) (° C.)(mm) (%) Comparative A 1196 2.4 955  58 556 1.4 42 Example 1 ComparativeB 1210 2.5 935 661 545 1.6 36 Example 2 Comparative C 1203 1.8 932  48607 0.9 50 Example 3 Comparative D 1221 2.0 942  35 633 1.0 50 Example 4Comparative E 1244 2.4 938  55 522 1.4 42 Example 5 Comparative F 11892.1 966  56 525 1.2 43 Example 6 Comparative G 1202 2.3 952  47 565 1.439 Example 7 Comparative H 1234 2.5 922  55 545 1.5 40 Example 8Comparative I 1231 2.5 925  61 567 1.6 36 Example 9 Comparative J 11982.7 945  66 552 1.8 33 Example 10 Inventive K 1212 2.2 949  58 555 1.245 Example 1 Inventive L 1248 2.1 930  57 565 1.2 43 Example 2Comparative M 1245 2.5 947  49 552 1.6 36 Example 11

TABLE 3 Average Average cooling cooling rate for rate for primarysecondary Reheating Steel SS cooling SCS cooling RCS RHS rateClassification type (° C.) (° C./s) (° C.) (° C./s) (° C.) (° C.) (°C./s) Comparative A 859 2.6 685 19.2 306 421 0.7 Example 1 Comparative B861 3.9 637 18.6 315 418 1.4 Example 2 Comparative C 862 3.9 620 16.1323 417 0.7 Example 3 Comparative D 857 4.2 577 12.9 322 445 0.9 Example4 Comparative E 846 3.9 605 15.5 319 431 1.8 Example 5 Comparative F 8474.4 595 17.0 301 422 0.9 Example 6 Comparative G 860 5.0 587 17.8 297424 2.6 Example 7 Comparative H 847 3.7 622 17.4 302 413 0.8 Example 8Comparative I 847 2.4 675 15.6 343 417 0.6 Example 9 Comparative J 8573.4 633 14.5 347 422 1.2 Example 10 Inventive K 840 4.0 607 17.6 302 4161.5 Example 1 Inventive L 848 3.6 612 16.1 295 415 1.8 Example 2Comparative M 842 4.1 606 17.6 301 445 3.1 Example 11

TABLE 4 Fraction Fraction Fraction Fraction Maximum of retained of freshof of LME Steel YS TS EL HER austenite martensite ferrite cementitecrack Classification type (MPa) (MPa) (%) YR (%) (area %) (area %) (area%) (volume %) (μm) Comparative A  940 1126 10.8  0.83 32.7 5%  5% 1% 1 80 Example 1 Comparative B  962 1102 10.4  0.87 33.7 4%  3% 0% 2  69Example 2 Comparative C  898 1139 9.7 0.79 22.1 4% 15% 3% 2  60 Example3 Comparative D  959 1086 11.3  0.88 51.1 5%  0% 2% 2  87 Example 4Comparative E  978 1152 11.3  0.85 28.4 5%  5% 0% 1  77 Example 5Comparative F  963 1232 9.6 0.78 22.4 5% 10% 1% 1 107 Example 6Comparative G  744 1304 10.0  0.57 10.9 4% 20% 4% 1  76 Example 7Comparative H 1092 1253 12.2  0.87 25.6 6%  8% 0% 1  65 Example 8Comparative I  779 1239 10.6  0.63 17.2 7% 19% 2% 1  71 Example 9Comparative J  917 1193 9.8 0.77 16.6 5% 17% 1% 1  77 Example 10Inventive K  919 1181 11.0  0.78 36.5 4%  5% 0% 2  66 Example 1Inventive L  839 1235 11.3  0.68 37.9 5% 10% 1% 2  69 Example 2Comparative M 1042 1195 15.2  0.87 41.1 9%  3% 0% 0 149 Example 11

First, Comparative Examples 1 to 5 are cases in which steel types A to Ewere applied, respectively. Steel types A to E have the contents of C,Mn, or Cr lower than that of the range of the present disclosure, inwhich strength of TS 1180 MPa class could not be obtained. Even forsteels, like steel types A to E, in which the alloy component additionamount is outside of the range of the components of the presentdisclosure, tensile strength higher than 1180 MPa may be obtained ifannealing heat treatment conditions are significantly changed, but inthis case, it is necessity to introduce an excessively large amount offresh martensite and a high hole expansion ratio cannot be obtained.Comparative Example 6 is a case to which steel type F having a C contentexceeding the range of the present disclosure was applied, in which ahigh hole expansion ratio could not be obtained even if the processconditions suggested in the present disclosure were satisfied.

Steel type G of Comparative Example 7 is a case in which the Mn contentexceeds the range of the present disclosure, whereby a ratio of freshmartensite reaches 20%, so that a hole expansion ratio is significantlydeteriorated and a yield ratio is also too low. In addition, steel typeH of Comparative Example 8 was a steel type in which Cr was increasedinstead of Mn, and it was difficult to obtain a low yield ratio.

To Comparative Examples 9 and 10, steel types I and J satisfying thealloy composition of the present disclosure were applied, but as anannealing and quenching temperature exceeded 330° C., the ratio of freshmartensite increased and a hole expansion ratio was significantlydeteriorated.

Inventive Examples 1 and 2 are cases to which steel types K and Lsatisfying the alloy composition of the present disclosure are appliedand in which all process conditions are satisfied, and here, a holeexpansion ratio of 25% or more and elongation suitable for processing of5% to 13% may be obtained at a low yield ratio of 0.65 to 0.85.

Steel types F and M applied to Comparative Examples 6 and 11,respectively, have an alloy amount that does not satisfy Equation 1, anddue to this, it can be seen that a maximum size of the LME crack in theweld portion exceeded 100 μm, and thus, LME crack resistance wasinferior.

Meanwhile, cracks in an overlapping portion, which are not allowed toexist as severe LME cracks, were not present in all of the testmaterials.

While exemplary embodiments have been shown and described above, it willbe apparent to those skilled in the art that modifications andvariations could be made without departing from the scope of the presentdisclosure as defined by the appended claims.

1. A high-strength cold rolled steel sheet comprising: by weight percent(wt %), 0.17 to 0.21% of carbon (C), 0.3 to 0.8% of silicon (Si), 2.7 to3.3% of manganese (Mn), 0.3 to 0.7% of chromium (Cr, 0.01 to 0.3% ofaluminum (Al), 0.01 to 0.03% of titanium (Ti), 0.001 to 0.003% of boron(B), 0.04% or less of phosphorus (P), 0.02% or less of sulfur (S), 0.01%or less of nitrogen (N), the balance of iron (Fe), and other inevitableimpurities, wherein the contents of carbon (C), silicon (Si), andaluminum (Al) satisfy Equation 1 below, a microstructure thereofincludes, by area fraction, 3 to 7% of retained austenite, 5 to 15% offresh martensite, 5% or less (including 0%) of ferrite, and the balanceof bainite or tempered martensite, and, by volume fraction, 1 to 3% of acementite phase, as a second phase, is precipitated and distributedbetween bainite laths or in the laths or grain boundary of a temperedmartensite phase,[C]+([Si]+[Al])/5≤0.35%  [Equation (1)] wherein [C], [Si], [Al] refer toweight percents of C, Si, and Al, respectively.
 2. The high-strengthcold rolled steel sheet of claim 1, wherein the cold rolled steel sheetfurther includes 0.1% or less of copper (Cu), 0.1% or less of nickel(Ni), and 0.1% or less of molybdenum (Mo).
 3. The high-strength coldrolled steel sheet of claim 1, wherein the cold rolled steel sheetfurther includes 0.03% or less of niobium (Nb) and 0.01% of less ofvanadium (V).
 4. The high-strength cold rolled steel sheet of claim 1,wherein the cold rolled steel sheet has a tensile strength of 1180 MPaor more, a yield ratio of 0.65 to 0.85, a hole expansion ratio of 25% ormore (HER), and an elongation of 5 to 13%.
 5. A high strength hot-dipgalvanized steel sheet comprising a hot-dip zinc plating layer on asurface of the high-strength cold rolled steel sheet of claim
 1. 6.(canceled)
 7. A method of manufacturing a high-strength cold rolledsteel sheet, the method comprising: preparing a slab including, byweight percent (wt %), 0.17 to 0.21% of carbon (C), 0.3 to 0.8% ofsilicon (Si), 2.7 to 3.3% of manganese (Mn), 0.3 to 0.7% of chromium(Cr, 0.01 to 0.3% of aluminum (Al), 0.01 to 0.03% of titanium (Ti),0.001 to 0.003% of boron (B), 0.04% or less of phosphorus (P), 0.02% orless of sulfur (S), 0.01% or less of nitrogen (N), the balance of iron(Fe), and other inevitable impurities, wherein the contents of carbon(C), silicon (Si), and aluminum (Al) satisfy Equation 1 below; heatingthe slab to a temperature in a range of 1,150° C. to 1,250° C.; finishhot rolling the heated slab within a finish delivery temperature (FDT)range of 900° C. to 980° C.; cooling the slab at an average cooling rateof 10° C./sec to 100° C./sec after the finish hot rolling; winding theslab in a temperature in a range of 500° C. to 700° C.; cold rolling theslab at a cold-rolling reduction ratio of 30% to 60% to obtain a coldrolled steel sheet; continuously annealing the cold rolled steel sheetat a temperature in a range of (Ae3+30° C. to Ae3+80° C.); primarilycooling the continuously annealed steel sheet at an average cooling rateof 10° C./s or less to a temperature in a range of 560° C. to 700° C.and secondarily cooling the steel sheet at an average cooling rate of10° C./s or more to a temperature in a range of 270° C. to 330° C.; andreheating the cooled steel sheet at a temperature increase rate of 5°C./s or lower to a temperature in a range of 380° C. to 460° C.,[C]+([Si]+[Al])/5≤0.35%  [Equation (1)] wherein [C], [Si], and [Al]refer to weight percent of C, Si, and Al, respectively.
 8. The method ofclaim 7, wherein the slab further includes 0.1% or less of copper (Cu),0.1% or less of nickel (Ni), and 0.1% or less of molybdenum (Mo).
 9. Themethod of claim 7, wherein the slab further includes 0.03% or less ofniobium (Nb) and 0.01% or less of vanadium (V).
 10. The method of claim7, wherein the continuous annealing is performed at a temperature in arange of 830° C. to 880° C. 11.-13. (canceled)